The present invention relates to a device for growing mollusks and in particular oysters out at sea.
The growing of oysters or oyster farming, such as that which has been known for a long time, consists of collecting in the natural medium the larvae coming from the laying by the parents that become fixed on supports or collectors arranged in suitable regions by the oyster farmers.
When they have reached a sufficient size, the oysters are separated from the collectors and replaced or sown in the natural medium so that they can grow therein until they reach a size suitable for their commercialization.
In order to be able to sow and harvest the oysters, for many years the oysters were sown on an oyster park, i.e. places which are uncovered at low tide.
This growing method requires many tiresome operations and has an artisanal aspect, the yield being low owing to the necessity of these many manual operations and access to the parks during only limited periods.
In some regions, the oysters are sown in places which are not uncovered at low tide and the harvesting is effected with a dredge.
Such a method does not permit a supervision of the evolution oysters.
In recent years a much more industrial method has been developed, for example as concerns the collecting of the larvae which is effected in enclosed hatching grounds. These larvae are thereafter treated and monitored in a scientific manner until they reach a size sufficient to put them back in the natural medium.
In this last stage, the oysters are no longer sown but disposed on circular trays arranged inside a net of tubular shape, the trays being vertically evenly spaced apart and thereby constituting what is conventionally termed a Japanese lantern, the assembly having the shape of such an object.
The Japanese lanterns are suspended in spaced-apart relation from a hawser anchored in deep water and provided with floats.
Such Japanese lanterns have an overall height of 5 meters and their installation requires the use of a relatively large boat provided with handling and hoisting means such as for example a crane.
Presently-known trays are circular trays having an apertured central part provided with stiffeners.
Outside the central part there extends an annular edge portion whose periphery comprises parts similar to the tip of an arrow and evenly spaced apart, connecting parts between two tips being formed by a cylindrical groove perpendicular to the plane of the tray and outwardly open between two adjacent tips for receiving a thread of the net.
Further, arranged on the radial end of each tip is a point extending downwardly in a direction perpendicular to the plane of the tray so as to be capable of radially retaining a thread of the net which is generally substantially perpendicular to the thread retained in the aforementioned groove.
The annular edge portion comprises three apertures extending in a direction perpendicular to the plane of the tray and evenly spaced apart.
The construction of a Japanese lantern with such a tray according to the prior art will now be described, which will permit explanation of the drawbacks of the tray.
In a first stage, the trays are arranged along three ropes which pass through the apertures in the annular edge portion, the trays being evenly spaced apart along the ropes by retaining means, for example knots.
The trays are stacked one on top of the other in a metal sleeve, the ropes being folded between the trays and their upper ends being fixed to a hoisting hook.
In a second stage, the tubular-shaped net is folded in the form of an accordeon around the metal sleeve, three cables being fixed at 120.degree. to one another for example in the region of the open upper circumference of the net and connected at the aforementioned hoisting hook.
When this has been achieved, the hoisting hook is hoisted about 15 to 20 centimeters, which corresponds to the spacing between two trays when the cable parts located therebetween are taut, which causes the upper part of the net to extend above the sleeve and the upper tray. In this position, the upper tray is hooked to the interior of the net by means of points or lugs and grooves.
The operation is repeated until all of the plates are hooked to the interior of the tubular-shaped net, after which the lower end of the net is gathered under the lowest tray and provided with ballast.
Lanterns constructed in this way are hooked to a hawser adapted to be cast into the sea in the known manner, the lanterns being evenly spaced apart.
It has been found that in constructing a lantern with known trays in the manner described hereinbefore, the tray is not always horizontal since the hooking to the net is not achieved in a reliable manner.
A manual modification of the hooking may be required, which takes time and is expensive and does not always provide a sufficient horizontality so that the young oysters become heaped on a sector of the tray and cannot grow in the required manner and this considerably reduces the yield.
Further, the Japanese lanterns swing in the water under the effect of currents and, when they are too close to each other, one tray of a lantern may hook by means of its points or lugs onto the net of an adjacent lantern. In this case, the nets become torn and consequently a large amount of the product is lost in the course of growth, which again reduces the yield.
In order to avoid this last drawback, the lanterns, which have an effective height of about 3 meters, are spaced apart at least two meters on the hawser.